Fabrication of integrated circuitry frequently utilizes photolithography to define structures. Specifically, radiation is passed through a photomask to pattern the radiation into light and shadow regions. The patterned radiation is utilized to impart an exposure pattern to photosensitive material (photoresist). The photoresist is then subjected to a developing solution. The developing solution selectively removes regions of the resist exposed to light relative to regions exposed to shadow, or vice versa (depending on whether the photoresist is a positive resist or a negative resist), and thus transforms the exposure pattern into a physical pattern formed in the photoresist.
It is noted that photomasks may alternatively be referred to as reticles. Historically, there has been some distinction between the terms photomask and reticle (with the term photomask referring to masks that form a pattern extending across an entirety of the substrate, and the term reticle referring to masks that form a pattern extending only partially across a substrate), but the distinction has become blurred in modern usage of the terms. Accordingly, the terms photomask and reticle are utilized interchangeably in this disclosure to refer to masks that may form patterns extending across an entirety of substrate, or across only a portion of a substrate.
Integrated circuitry may comprise multiple levels of structures stacked over a semiconductor substrate. The integrated circuitry may also comprise a variety of different structures formed within each level. Numerous photomasks may be utilized to create the different structures within a level, and to create the different levels. There is a risk of mask misalignment every time a new photomask is introduced into a fabrication sequence. Also, there is process time associated with each photomask which slows overall throughput of a fabrication process.
A continuing goal of semiconductor fabrication is to reduce photomasking steps in order to avoid mask misalignment and improve throughput.
Another aspect of the prior art is that non-volatile devices (flash) may be utilized in numerous memory and logic applications of integrated circuits. The term “flash” historically referred to particular non-volatile devices programmed with flashes of radiation. The term has become generic for any non-volatile structure utilizing a control gate and a floating gate, and will be utilized with such generic meaning throughout this disclosure.
It is desired to develop improved methods for forming flash, and for such methods to utilize only one photomasking step for patterning of multiple different structures.